Philipp made an interesting observation this morning about the difference between Google AdWords and AdSense units -- the clickable zones on them are noticeably different enough to raise some concern. It appears AdSense units are designed for accidental clicks -- or at least not designed for no accidental clicks.
A long time ago, Google made the ad units on their result pages only respond to clicks on the title link -- before that, you could click anywhere on the advertisement. Why did they stop doing this? Here's what they said:
"these changes help decrease the likelihood that a user will unintentionally click on an ad"
Now the question is, even after updates to AdSense, do they still allow "accidental" clicks? It seems to me this is being done in their best interest and at the same time ripping off a lot of advertisers who choose to show their ads in the AdSense network.